The invention concerns a process for cooling hot bulk materials in open coolers with gas-permeable cooling buckets moving on a straight or circular track, whereby a gaseous cooling media is forced through the material resting in the buckets.
It is the purpose of the process and apparatus of the present invention to reduce the dust nuisance occuring near cooling apparatus of this type. Such cooling apparatus are well known for different materials, especially sintering cooling apparatus, wherein a gaseous media is forced through the material disposed in the cooling buckets. The magazine Stahl Und Eisen 87, 1967, No. 24, Pages 1472-1477, contains a description of such a cooling apparatus. The cooling apparatus described in the above mentioned article has a circular track with ten cooling air supply ducts in ten separate identical blowers disposed about the circumference of the cooling apparatus. The relatively cool ambient air sucked in by the blowers penetrates the material from below or from the side of the buckets and passes through the bulk material. A uniform volume of cooling air is passed through the hot bulk material along the entire length of the cooling apparatus. The air, heated during penetration of the hot material, carries dust particles from the material into the surrounding area resulting in a severe nuisance. It has been noticed that the ejected quantity of dust or particulate emission is 50 to 100 times greater at the charging end of the cooling apparatus than at the discharge end. The operator of such a cooling apparatus is forced to collect the emitted dust directly above the cooling buckets for separation in an electrostatic or mechanical dust removal apparatus. The abrasive and corrosive dust particles of the sinter plant can cause considerable wear in the duct work. It is the object of this invention to reduce the dust emission from the bulk material during the cooling process, and to obviate the need for any additional dust collector. This is achieved by increasing the volume of cooling media per unit area which is forced through the hot bulk material from the charging end of the cooling apparatus to the discharge end of the cooling apparatus. The hot bulk material as it is charged to the cooling apparatus has a temperature of about 700.degree. to 900.degree.C, and a reduced volume of cooling air is forced through the material at the charging end than at the discharge end of the cooling apparatus. Due to the large temperature difference between the hot bulk material which is at 700.degree. to 900.degree.C and the ambient cooling air which is at from 10.degree. to 30.degree.C, good heat exchange is achieved even with the reduced cooling air volume and cooling air velocity penetrating the bulk material. The bulk material is cooled down to a temperature of about 80.degree. to 100.degree.C proximate the discharge end of the cooling apparatus. While there is a low temperature differential between the material at the discharge end and the cooling air, the resulting lower rate of heat exchange is offset by the higher volume of cooling air and the resultant increased flow through the material layer. Surprisingly, it has been discovered that at the same cooling capacity the amount of dust emitted from the bulk material proximate the charging end of the cooling apparatus can be remarkably reduced and is more uniformly distributed over the entire travel path of the cooling apparatus when the volume of cooling air is reduced proximate the charging zone. It has also been discovered that the particle size of the dust which is evolved proximate the discharge end using the present invention is considerably smaller than the average particle size of dust evolved from a prior art cooling apparatus. The volume of cooling air forced through the bulk material can be increased over the length of the cooling apparatus, or distinct zones can be defined wherein the cooling air volume is increased in steps. The regulation of the cooling air directed through the bulk material is had by suitable design for the plenum chamber beneath the cooling bucket.
In practicing the present invention it is desirable that the volume of cooling air passed through the hot bulk material proximate the charging end of the cooling apparatus be about 30 to 70%, and preferably 50 percent of the volume of cooling air forced through the material proximate the discharge end of the cooling apparatus. Such operation will reduce the dust content above the cooling buckets substantially.
The preferred cooling apparatus for carrying out this process includes open cooling buckets travelling on straight or circular tracks whereby a gaseous media penetrates the cooling buckets. At least one throttling means is disposed in a gas-tight plenum chamber located beneath the cooling buckets and communicating therewith, and at least one blower means is connected to the plenum chamber, preferably proximate the discharge end of the cooling apparatus. The throttling means in the plenum chamber is used to adjust the volume of cooling media which is forced through the bulk material at various zones along the travel length of the cooling apparatus. The blower means which can be a single blower or a plurality of blowers can then be arranged proximate the discharge portion of the cooling apparatus to increase the volume of cooling media flowing through the bulk material from the charging end to the discharging end. Such an apparatus eliminates the need for many individual blowers and gas ducts as described in the aforementioned prior art apparatus. The apparatus of the present invention employs a simplified design for the plenum chamber which permits an elimination of duplicative blowers and duct work.
In the embodiment of the present invention the throttling means comprises a partition wall having a variable orifice therethrough. The use of such a partition wall with a variable orifice permits accurate regulation of the flow of cooling media to minimize the dust emission. In another embodiment, the plenum chamber can be divided up by a plurality of partition walls having variable orifices resulting in three distinct zones of different lengths, and preferably having a length ratio of 1:2:7 respectively for the first zone proximate the charging end, for the second intermediate zone, and for the third zone proximate the discharge end of the cooling apparatus. A significant reduction in the amount of evolved dust can thus be achieved.